Fibrocytes
Inflammation is the coordinated response to tissue injury or infection. The initiating events are mediated by local release of chemotactic factors, platelet activation, and initiations of the coagulation and complement pathways. These events stimulate the local endothelium, promoting the extravasation of neutrophils and monocytes. The second phase of inflammation is characterized by the influx into the tissue of cells of the adaptive immune system, including lymphocytes. The subsequent resolution phase, when apoptosis of the excess leukocytes and engulfment by tissue macrophages takes place, is also characterized by repair of tissue damage by stromal cells, such as fibroblasts.
In chronic inflammation, the resolution of inflammatory lesions is disordered, with the maintenance of inflammatory cells, fibroblast hyperplasia, and eventual tissue destruction. The mechanisms that lead to these events are complex, but include enhanced recruitment, survival and retention of cells and impaired emigration.
The source of fibroblasts responsible for repair of wound lesions or in other fibrotic responses is controversial. The conventional hypothesis suggests that local quiescent fibroblasts migrate into the affected area, produce extracellular matrix proteins, and promote wound contraction or fibrosis. An alternative hypothesis is that circulating fibroblast precursors (called fibrocytes) present within the blood migrate to the sites of injury or fibrosis, where they differentiate and mediate tissue repair and other fibrotic responses.
Fibrocytes are fibroblast-like cells that appear to participate in wound healing and are present in pathological lesions associated with, inter alia, asthma, pulmonary fibrosis and scleroderma. Fibrocytes are known to differentiate from a CD14+ peripheral blood monocyte precursor population. Fibrocytes may also differentiate from other sources. Fibrocytes express markers of both hematopoietic cells (CD45, MHC class II, CD34) and stromal cells (collagen types I and III and fibronectin). Fibrocytes at sites of tissue injury secrete inflammatory cytokines, extracellular matrix proteins and promote angiogenesis and wound contraction. Fibrocytes are also associated with the formation of fibrotic lesions after infection or inflammation, and are implicated in fibrosis associated with autoimmune diseases.
Control of fibrocyte differentiation is likely to be important in the control of many diseases and processes. Fibrocytes are associated with a variety of processes and diseases including scleroderma, keloid scarring, rheumatoid arthritis, lupus, nephrogenic fibrosing dermopathy, and idiopathic pulmonary fibrosis. They play a role in the formation of fibrotic lesions after Schistosoma japonicum infection in mice and are also implicated in fibrosis associated with autoimmune diseases. Fibrocytes have also been implicated in pathogenic fibrosis, fibrosis associated with radiation damage, Lyme disease and pulmonary fibrosis. CD34+ fibrocytes have also been associated with stromal remodeling in pancreatitis and stromal fibrosis, whereas lack of such fibrocytes is associated with pancreatic tumors and adenocarcinomas. Fibrosis additionally occurs in asthma patients and possibly other pulmonary diseases such as chronic obstructive pulmonary disease when fibrocytes undergo further differentiation into myofibroblasts.
Fibrocytes may also play a role in a variety of conditions, likely even some in which fibrocyte formation is not currently known. Some additional conditions may include congestive heart failure, other post-ischemic conditions, post-surgical scarring including abdominal adhesions, corneal refraction surgery, and wide angle glaucoma trabeculectomy. Fibrocytes are also implicated in liver fibrosis and cirrhosis. See Tatiana Kisseleva et al, Bone Marrow-Derived Fibrocytes Participate in Pathogenesis of Liver Fibrosis, 45 Journal of Hepatology 429-438 (September 2006); see also F. P. Russo et al, The Bone Marrow Functionality Contributes to Liver Fibrosis, 130(6) Gastroenterology 1807-21 (May 2006). Fibrocytes are important in the formation of tumors, particularly stromal tissue in tumors. Recent evidence also suggests that fibrocytes may further differentiate into adipocytes and thus play a role in obesity.
It has been previously identified that fibrocytes may differentiate from CD14+ peripheral blood monocytes, and the presence of human serum dramatically delays this process. The factor in human serum that inhibits fibrocyte differentiation is serum amyloid P (SAP). SAP, a member of the pentraxin family of proteins that includes C-reactive protein (CRP), is produced by the liver, secreted into the blood, and circulates in the blood as stable pentamers. SAP binds to receptors for the Fc portion of IgG antibodies (FcγR) on a variety of cells and may effectively cross-link FcγR without additional proteins because SAP is a pentameric protein with five potential FcγR binding sites per molecule. As SAP binds to FcγR, intracellular signaling events consistent with FcγR activation are initiated.
Anti-FcγR Antibodies
It has also been identified that anti-FcγR antibodies may also prevent the differentiation of peripheral blood monocytes into fibrocytes. Anti-FcγR antibodies are IgG antibodies that bind to receptors for the Fc portion of IgG antibodies (FcγR). The anti-FcγR antibodies bind through their variable region, and not through their constant (Fc) region. However, IgG from the appropriate source (e.g. human IgG for human receptors) may normally bind to FcγR through its Fc region. FcγR are found on the surface of a variety of hematopoietic cells. There are four distinct classes of FcγR. FcγRI (CD64) is expressed by peripheral blood monocytes and binds monomeric IgG with a high affinity. FcγRII (CD32) and FcγRIII (CD16) are low affinity receptors for IgG and only efficiently bind aggregated IgG. FcγRII is expressed by peripheral blood B cells and monocytes, whereas FcγRIII is expressed by NK cells and a subpopulation of monocytes. FcγRIV was recently identified in mice and is present on murine peripheral blood monocytes and neutrophils, macrophages and dendritic cells and efficiently binds murine IgG2a and IgG2b antibodies. There is a putative human FcγRIV gene, but the biological function of the protein, such as ligand specificity and cellular expression is, as yet unknown.
Peripheral blood monocytes express both FcγRI and FcγRII (a subpopulation of monocytes express FcγRIII), whereas tissue macrophages express all three classical FcγR. Clustering of FcγR on monocytes by IgG, either bound to pathogens or as part of an immune complex, initiates a wide variety of biochemical events.
FcγR activation and induction of intracellular signaling pathways may occur when multiple FcγR are cross-linked or aggregated. This FcγR activation leads to a cascade of signaling events initiated by two main kinases. The initial events following FcγR activation involve the phosphorylation of intracellular immunoreceptor tyrosine activation motifs (ITAMs) present on the cytoplasmic tail of FcγRIIa or the FcR-γ chain associated with FcγRI and FcγRIII, by Src-related tyrosine kinases (SRTK). In monocytes, the main Src-kinases associated with FcγRI and FcγRII are hck and lyn. The phosphorylated ITAM then recruit cytoplasmic SH2-containing kinases, especially Syk, to the ITAMs and Syk then activates a series of downstream signaling molecules.
Anti-FcγR antibodies for FcγRI (anti-FcγRI) and for FcγRII (anti-FcγRII) are able to bind to either FcγRI or FcγRII, respectively. These FcγR may then be cross-linked by the binding of additional antibodies or other means. This process initiates intracellular signaling events consistent with FcγR activation.
Scleroderma
Scleroderma is a non-inherited, noninfectious disease that has a range of symptoms. It involves the formation of scar tissue containing fibroblasts in the skin and internal organs. The origin of the fibroblasts is unknown. In mild or early cases of scleroderma, there is a hardening of the skin, fatigue, aches and sensitivity to cold. In more severe and later stages, there is high blood pressure, skin ulcers, difficulty moving joints, and death from lung scarring or kidney failure. Approximately 300,000 people in the U.S. have scleroderma. The disease has similarities to lupus and rheumatoid arthritis. There is no cure or significant treatment for scleroderma and even diagnosis is difficult because there is no clinical test.
Nephrogenic Fibrosing Dermopathy
Nephrogenic fibrosing dermopathy (NFD) is a newly recognized scleroderma-like fibrosing skin condition. It develops in patients with renal insufficiency. Yellow scleral plaques and circulating antiphospholipid antibodies have been proposed as markers of NFD. Dual immunohistochemical staining for CD34 and pro-collagen in the spindle cells of NFD suggest that the dermal cells of NFD may represent circulating fibrocytes recruited to the dermis. Therefore, inhibition of fibrocyte formation may alleviate symptoms of this disease.
Asthma
Asthma affects more than 100 million people worldwide, and its prevalence is increasing. Asthma appears to be caused by chronic airway inflammation. One of the most destructive aspects of asthma is remodeling of the airways in response to chronic inflammation. This remodeling involves thickening of the lamina reticularis (the subepithelial reticular basement membrane surrounding airways) due to fibrosis. The airway passages then become constricted due to the thickened airway walls.
The thickened lamina reticularis in asthma patients contains abnormally high levels of extracellular matrix proteins such as collagen I, collagen III, collagen V, fibronectin and tenascin. The source of these proteins appears to be a specialized type of fibroblast called myofibroblasts.
In asthma patients, CD34+/collagen I+ fibrocytes accumulate near the basement membrane of the bronchial mucosa within 4 hours of allergen exposure. 24 hours after allergen exposure, labeled monocytes/fibrocytes have been observed to express α-smooth muscle actin, a marker for myofibroblasts. These observations suggest that in asthma patients allergen exposure causes fibrocytes from the blood to enter the bronchial mucosa, differentiate into myofibroblasts, and then cause airway wall thickening and obstruct the airways. Further, there is a correlation between having a mutation in the regulatory regions of the genes encoding monocyte chemoattractant protein 1 or TGFβ-1 and the severity of asthma. This also suggests that recruitment of monocytes and appearance of myofibroblasts lead to complications of asthma.
Thickening of the lamina reticularis distinguishes asthma from chronic bronchitis or chronic obstructive pulmonary disease and is found even when asthma is controlled with conventional medications. An increased extent of airway wall thickening is associated with severe asthma. No medications or treatments have been found to reduce thickening of the lamina reticularis. However, it appears likely that reducing the number of myofibroblasts found in the airway walls may reduce thickening or help prevent further thickening.
Idiopathic Pulmonary Fibrosis
Idiopathic pulmonary fibrosis (IPF) is a unique type of chronic fibrosing lung disease of unknown etiology. The sequence of the pathogenic mechanisms is unknown, but the disease is characterized by epithelial injury and activation, the formation of distinctive subepithelial fibroblast/myofibroblast foci, and excessive extracellular matrix accumulation. These pathological processes usually lead to progressive and irreversible changes in the lung architecture, resulting in progressive respiratory insufficiency and an almost universally terminal outcome in a relatively short period of time. While research has largely focused on inflammatory mechanisms for initiating the fibrotic response, recent evidence strongly suggests that disruption of the alveolar epithelium is an underlying pathogenic event. Given the role played by fibrocytes in wound healing and their known role in airway wall thickening in asthma, it appears likely that overproduction of fibrocytes may be implicated in IPF.